1. Field of the Invention
Embodiments of this invention relate to the excitation of switched reluctance motors. In particular, they relate to excitation to reduce the supply current drawn for a particular output.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21–24 Jun. 1993, incorporated herein by reference. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be rectified AC mains, a battery or some other DC source. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms and its output may also be used to generate a speed feedback signal.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
For any system, there is the requirement to know the position of the rotor relative to the stator. While a high-resolution resolver could be used, these are relatively expensive and unnecessarily complex for the majority of applications. Instead, a relatively simple rotor position transducer (RPT) is usually used, comprising a castellated member fixed to the rotor and a set of detectors fixed relative to the stator. Common systems use disc or cup-shaped vanes on the rotor with the same number of teeth as there are rotor poles, together with optical or magnetic detectors on the stator, typically with one detector for each phase of the drive. Such a system is illustrated in FIG. 5 for a 3-phase system with four rotor poles.
The switched reluctance drive is essentially a variable speed system and is characterized by voltages and currents in the phase windings of the machine which are quite different from those found in traditional, sinusoidally fed, types of machines. As is well known, there are two basic modes of operation of switched reluctance systems: the chopping mode and the single-pulse mode, both of which are described in the Stephenson paper cited above. FIGS. 3(a)–3(c) illustrate typical waveforms in single-pulse control. FIG. 3(a) shows the voltage waveform applied by the controller to the phase winding. At a predetermined rotor angle, the voltage is applied by switching on the switches in the power converter 13 and applying constant voltage for a given angle θc, the conduction angle. The current rises from zero, reaches a peak and falls slightly as shown in FIG. 3(b). When θc has been traversed, the switches are opened and the action of energy return diodes places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then typically a period of zero current until the cycle is repeated. It will be clear that the phase is drawing energy from the supply during θc and returning a smaller amount to the supply thereafter. FIG. 3(c) shows the current which has to be supplied to the phase winding by the power converter and the current which flows back to the converter during the period of energy return. Instead of opening both switches simultaneously, it is well known that there are advantages in opening one switch in advance of the other, allowing the current to circulate around the loop formed by the closed switch, the phase winding and a diode—this is known as “freewheeling” and is used for various reasons, including peak current limitation and acoustic noise reduction. The single-pulse mode is normally used for the medium and high speeds in the speed range of a typical drive.
At zero and low speeds, however, the single-pulse mode is not suitable, due to the high peak currents which would be experienced, and the chopping mode is used. There are two principal variants of the chopping mode. The simplest method is to simultaneously open the two switches associated with a phase winding, e.g. switches 21 and 22 in FIG. 2. This causes energy to be returned from the machine to the DC link, with a corresponding rapid fall in the phase current. This is sometimes known as “hard chopping”. The alternative method is to open only one of the switches and allow freewheeling to occur: this is known as “freewheel chopping” or “soft chopping”. In this mode of control, no energy is returned to the DC link from the phase winding.
With any chopping scheme, there is a choice of strategy for determining the current levels to be used. Many such strategies are known in the art. One commonly used scheme is to use a hysteresis controller which enables chopping between upper and lower currents. A typical scheme is shown in FIG. 4(a) for hard chopping. At a chosen switch-on angle θon (which is often the position at which the phase has minimum inductance, but may be some other position), the voltage is applied to the phase winding and the phase current is allowed to rise until it reaches the upper hysteresis current Iu. At this point both switches are opened and the current falls until it reaches the lower current Il and the switches are closed again, repeating the chopping cycle until a chosen switch-off angle θoff is reached. FIG. 4(b) shows the corresponding phase current waveform for a hysteresis controller using freewheeling: the reduction in chopping frequency is immediately evident.
While switched reluctance motors are described here in relation to current control and, implicitly, its feedback to the controller, those of ordinary skill in the art will appreciate that the output of a switched reluctance motor can be subjected to flux control instead. Indeed, flux has a more direct relationship to output torque or force and can, therefore, be a more accurate characteristic on which to base motor control.
None of the above control strategies takes into account what happens when the contributions of two or more phases are considered. In this situation the bus currents associated with the individual phases are added to give the total DC link current.
Two or more phases conducting together can occur in many different systems. Although in 2-phase systems it is usual to only operate the phases alternately, U.S. Pat. No. 5,747,962, commonly assigned to the present assignee and incorporated herein by reference, discloses a method of operating both phases simultaneously over part of the electrical cycle of the machine. In 3-phase machines, it is possible to operate by exciting Phase A alone, then Phase B alone, then Phase C alone. This pattern of energization is known variously in the art as 33% conduction (because one phase conducts for only 33% of a cycle) and 1-phase conduction (because only one phase is conducting at any one time). However, to improve both the minimum torque and the average torque output of the machine, advantage is often taken of the fact that the torque productive portions of each phase cycle overlap. Thus, an excitation pattern of A, AB, B, BC, C, CA, A . . . is often used. This pattern is described variously in the art as 50% conduction (since each phase conducts for 50% of its cycle) or 1½ phase conduction (since on average over the cycle there are 1½ phases conducting). Similarly for 4-phase machines, there are normally always two phases producing torque in the required direction, so phases can be energized in pairs: AB, BC, CD, DA, AB . . . Corresponding rules apply for higher phase numbers, in which it is possible to use three or more phases for at least part of the electrical cycle.
For a given excitation level, operation in this manner considerably increases the burden on the supply. For example, for a 3-phase system the average current demand is up 50% and in a 4-phase system is up 100%. While in some applications the absolute size of the DC link current is secondary to torque output, in other applications there is extreme sensitivity to the DC link current, as the source may have limited capacity. Typically, such limitations are found in sites which have stand-alone generation, or in mobile situations such as automotive, marine or aerospace applications. These sites are unsuited to drives in which there is simultaneous conduction by two phases because of the increased current demand.
Another way to address this problem is disclosed in EPA 1265349, commonly assigned to the present assignee and incorporated herein by reference. This describes a method of freewheeling in an outgoing phase while energizing the incoming phase from the supply. This attempts to provide the torque profile of a 1½ phase excitation while supplying current only to one phase. This works well when the inertia is low, but when the inertia is high the rotor accelerates slowly. In the meantime, the current in the freewheeling phase decays as a function of time, not rotor position, so there is a significant risk that the current (and hence the torque) will decay to zero before the rotor has reached a position where the next phase to be energized can supply sufficient torque on its own. If this happens, the motor will stall and can only be re-started if the outgoing phase is primed to re-establish the freewheeling current.
U.S. Pat. No. 5,539,293, commonly assigned to the present assignee and incorporated herein by reference, discloses a control system for starting a switched reluctance motor in which phases are excited in sequence in open loop with no reference to rotor position at all. This prior art is directed to zero load starting.